1. Field of the Invention
An aspect of this disclosure relates to a semiconductor integrated circuit, a protection circuit, and a battery pack.
2. Description of the Related Art
Portable devices driven by a rechargeable secondary battery are widely used. Such a secondary battery is often provided as a battery pack including a protection circuit. The protection circuit, for example, includes a function to protect the secondary battery from overcharge and overdischarge, and a function to monitor and manage the remaining energy level of the secondary battery.
FIG. 1 is a circuit diagram illustrating an exemplary configuration of a related-art protection circuit 10. As illustrated in FIG. 1, the protection circuit 10 includes a protection integrated circuit (IC) 11, a battery monitoring IC 12, a switching transistor M1, and a switching transistor M2. The protection circuit 10 further includes a terminal B+, a terminal B−, a terminal P+, and a terminal P−. A secondary battery B1 is connected between the terminal B+ and the terminal B−, and a charger or a load (not shown) is connected between the terminal P+ and the terminal P−. The protection IC 11 and the battery monitoring IC 12 are connected to each other to enable communications.
The protection IC 11 is driven by a voltage VDD supplied from the secondary battery B1. The protection IC 11 includes an N-type substrate. When detecting overcharge of the secondary battery B1 based on a voltage between VDD and VSS terminals (hereafter referred to as a “VSS-VDD voltage”), the protection IC 11 outputs a control signal from a terminal COUT to turn off the switching transistor M1 and thereby stop charging the secondary battery B1. Meanwhile, when detecting overdischarge of the secondary battery B1 based on the VSS-VDD voltage, the protection IC 11 outputs a control signal from a terminal DOUT to turn off the switching transistor M2 and thereby stop discharge from the secondary battery B1.
The battery monitoring IC 12 is supplied with a voltage generated from the voltage VDD by a regulator of the protection IC 11, and monitors the status of the secondary battery B1. The status of the secondary battery B1 includes, for example, the remaining energy level of the secondary battery B1 and a history of abnormal states of the secondary battery B1.
FIG. 2 is a circuit diagram illustrating an exemplary configuration of the protection IC 11. The protection IC 11 includes a reference voltage generating circuit 13, a comparator 14, a logic circuit 15, a regulator 16, a resistor R1, and a resistor R2.
The resistors R1 and R2 divide the VSS-VDD voltage. The comparator 14 compares the divided VSS-VDD voltage with a reference voltage generated by the reference voltage generating circuit 13, and outputs the comparison result to the logic circuit 15. The logic circuit 15 detects overcharge or overdischarge based on the comparison result from the comparator 14, and outputs a control signal from the terminal COUT or the terminal DOUT according to the detection result.
The reference voltage generated by the reference voltage generating circuit 13 is also supplied to the regulator 16. The regulator 16 includes an amplifier 17, a resistor R3, and a resistor R4. The reference voltage from the reference voltage generating circuit 13 is supplied to one of the input terminals of the amplifier 17. The other one of the input terminals of the amplifier 17 is supplied with an output voltage of the amplifier 17 itself. Also, the output voltage of the amplifier 17 is divided by the resistors R3 and R4, and the divided voltage is output from the regulator 16.
To charge the secondary battery B1, a charger is connected between the terminals P+ and P−. Here, it sometimes happens that the charger is connected in the reverse direction to the secondary battery B1 such that the positive and negative electrodes of the charger are connected to the opposite electrodes of the secondary battery B1. Hereafter, this connection state is referred to as a “reverse charger connection state”. In the reverse charger connection state, an excessive voltage is applied to the negative electrode of the secondary battery B1, which is an abnormal state for the secondary battery B1. For this reason, technologies have been proposed to prevent occurrence of the reverse charger connection state.
For example, Japanese Laid-Open Patent Publication No. 2009-247100 discloses a technology to stop charging a secondary battery when the reverse charger connection state is detected.
Referring to FIG. 1, when the reverse charger connection state occurs, the voltage at the terminal P− rises. As a result, the protection IC 11 detects a discharge overcurrent or a short circuit and turns off the transistor M2. However, since the substrate of the protection IC 11 is an N-type substrate whose substrate voltage is VDD, a parasitic transistor operates and a leak current is generated in the protection IC 11 when a terminal voltage becomes greater than VDD due to the reverse charger connection state. This leak current causes the output voltage of the regulator 16 to rise, and the output voltage may damage the battery monitoring IC 12.